The passport model is being used in cycling

Passports are an essential item for globetrotting tennis professionals. A biological passport could become an added carrying card.

Stuart Miller, who oversees tennis' anti-doping program for the International Tennis Federation, said Saturday such a program was "being considered seriously."

The so-called biological passport is a method of collecting and comparing biological data from athletes so that variances from normal biological levels or "markers" can be detected. Identifying those irregularities helps spot doping.

The passport model is being used in cycling and track and field.

"Our aim would be to implement it, but implementation is reasonably complex," Miller said by phone from London.

He characterized discussions within the ITF as "ongoing" rather than advanced but said it could be "fully operational in the order of 12 months" once it was underway.

Miller did not rule out implementation as early as 2014.

Consideration of the passport program coincides with a growing unease and outspokenness in the wake of Lance Armstrong's interview with Oprah Winfrey. Armstrong admitted to using EPO, testosterone and blood doping to fuel his record seven Tour de France wins.

Players from Serena Williams to Roger Federer said they were surprised and dismayed.

None was harsher than two-time defending men's champion Novak Djokovic.

"I think it's a disgrace for the sport to have an athlete like this," the No. 1 Serb said of Armstrong. "He cheated the sport. He cheated many people around the world with his career, with his life story. ... I think he should suffer for his lies all these years."

Others expressed concern about the efficacy of tennis' current program, especially the amount of blood testing it conducts away from competition.

According to the ITF's posted figures for 2011, there were only 21 out-of-competition blood tests.

At the ATP World Tour Championship in London, Federer and Scotland's Andy Murray called for more blood testing in the sport.

Djokovic said this week that he had not had a blood test in six or seven months. Bob Bryan, the top seed in doubles with brother Mike Bryan, said he had never had an out-of-competition blood test.

ESPN commentator and coach Darren Cahill said on Twitter that the amount of blood testing was "inadequate" and had "gone backwards."

"It's a proactive thing, not a reactive thing," said Cahill Sunday, adding that he believes the sport is clean. "But more funding is needed."

Some players said they would welcome a passport program.

"I'd be definitely in favor of that," Bob Bryan said Sunday after a third-round win against Jeremy Chardy and Lukasz Kubot. "Anything to make it harder to cheat."

For some, however, the sport is not doing nearly enough.

Guy Forget, once a top-5 player and now France's Davis Cup captain, said he believed he competed against players who were doping - though he has no proof.

He called for individual tournaments to divert prize money and put it toward more testing.

He is also favors a passport program, harsher penalties and preservation of blood samples so they can be retested in later years with more sophisticated methods.

"Some sports like cycling are more exposed, but we know in our sport some players have been caught," Forget said. "It would be foolish to not be concerned by these problems especially since the game got more physical."

The ITF's Miller could not put a dollar figure on a passport program for tennis but said "cost is an issue across all of anti-doping."

The ITF's anti-doping program, which is funded by the ATP and WTA and four Grand Slams, has been reported to be around $1.6 million.

Besides funding and informing stakeholders in the sport, Miller said establishing a passport program would require a period of sample gathering and an expert panel to review results.

"It's not just go and collect samples," he said. "We have to get all the pieces together before we're fully operational."

For the time being, players will probably continue to look over their shoulders.

Asked if she thought the sport was clean Saturday, two-time Grand Slam winner Svetlana Kuznetsova of Russia wasn't sure.